Conventionally, in alkaline primary batteries, a sealing agent is applied to a seal portion, i.e., between the battery case and the gasket disposed at the opening portion of the battery case, to improve leakage-resistance. The leakage in alkaline primary batteries is caused by creeping of the alkaline electrolyte, or by hydrogen gas generation under over-discharge or self-discharge.
Various examinations have been conducted on such a sealing agent to improve the leakage-resistance. For example, Patent Document 1 has proposed forming a coating of a sealing agent by applying and drying a sealing agent comprising asphalt, polybutene, and an organic solvent to the seal portion of alkaline primary batteries, and afterwards, heating to soften and melt the asphalt. Patent Document 2 has proposed applying a sealing agent containing polybutene to the seal portion of manganese dry cell batteries.
Patent Document 3 has proposed dissolving a sealing material containing polybutene in an organic solvent, and applying it to the seal portion of alkaline batteries. Patent Document 4 has proposed a sealing agent containing an aromatic hydrocarbon and polybutene with an average molecular weight of 700 to 10000 in a ratio of 10:90 to 80:20. In the field of fuel cells, a sealing agent using a hot-melt resin has been developed for preventing gas leakage.
However, the asphalt contained in the sealing agent of Patent Document 1 is solid at room temperature, and to form a uniform coating of a sealing agent at the seal portion with polybutene mixed therein, the asphalt has to be dissolved in a highly volatile organic solvent for the application to the seal portion. In Patent Document 3 as well, an organic solvent is used for uniformly applying the sealing agent to the seal portion, and probably this is also a highly volatile organic solvent which is easily dried. Highly volatile organic solvents are harmful to environment and human body, and have been a subject of legal restrictions in Europe nowadays. The highly volatile organic solvent mentioned herein refers to an organic solvent with a vapor pressure at 20° C. of 10 Pa or more.
On the other hand, polybutene contained in the sealing agent in Patent Document 1 is liquid at room temperature and has high flowability, and therefore it is difficult to be applied uniformly to the seal portion. Also, with repetitive rapid temperature changes, sometimes it flows out from the seal portion.
Further, the sealing agent of Patent Document 4 comprises a mixture of polybutene with a molecular weight of 700 to 10000, and a liquid-state aromatic hydrocarbon with a relatively low viscosity at room temperature. The mixture is excellent in flowability and application performance. However, since this sealing agent is liquid at room temperature, the sealing agent is not sufficiently dried, and with repetitive rapid temperature changes, the sealing agent sometimes flows out from the seal portion of the battery.
In the case when the above hot-melt resin is singly used as the sealing agent, unless the temperature is significantly raised for application, the flowability necessary for the application cannot be obtained sufficiently, and therefore uniform application is difficult. Additionally, since it completely solidifies after it is applied and dried, with repetitive rapid temperature changes, the battery case and gasket undergo expansion and contraction, which create gaps between the coating, and the battery case and the gasket, easily reducing sealing performance. Further, when the sealing agent is rubber, the sealing performance easily declines at low temperature.    [Patent Document 1] Japanese Laid-Open Patent Publication No. Sho 57-163962    [Patent Document 2] Utility Model Registration No. 2575986    [Patent Document 3] Japanese Laid-Open Patent Publication No. 2004-311069    [Patent Document 4] Japanese Laid-Open Patent Publication No. 2006-19178